refactor: enhance Dockerfile with build args and optimized caching

[ABGEO]

What's changing

Refactored the Dockerfile to make it easier to maintain and faster to build.

Why

• Build args let us update versions without editing the whole file
• Restored the mod cache layer so rebuilds are faster when you only change code
• Using the same Alpine version everywhere keeps things consistent

What I did

• Added GO_VERSION and ALPINE_VERSION as build args
• Re-added the go mod download step (better caching)
• Used the Alpine arg in the final stage instead of hardcoding
• Cleaned up spacing for readability